Techniques have been developed in the prior art to form thin-walled polytetrafluoroethylene (PTFE) tubes. For example, U.S. Pat. No. 4,925,710 to Buck et al., discloses a method of coating a core with PTFE, wherein, after sintering, the PTFE coating is removable from the core to form a tube. Automotive cables and medical catheters are disclosed as useful products. No disclosure is provided for uniformity of wall thickness nor the production of the tubes in the absence of the core. Tubes having wall thicknesses of 0.002 inches (0.051 mm) and less, which are formed on solid cores, are disclosed.
Other techniques have been developed in the prior art where porous expanded PTFE (ePTFE) is prepared in tape or film form, and rolled or wrapped into tubular form with overlapping edges being bonded and/or sintered. See, e.g., U.S. Pat. No. 4,791,966 to Eilentropp (discloses wrapping a PTFE tape about a mandrel); U.S. Pat. No. 6,027,779 to Campbell et al. (discloses overlaying two pieces of ePTFE film in forming a tube). Although these methods have produced thin-walled tubes having wall thicknesses of 300 μm and less, these thin-walled tubes are formed of ePTFE and are not susceptible to further expansion (and thus further wall reduction). In addition, these tubes include one or more seams.
With typical prior art PTFE tube extrusion, a PTFE billet is pre-formed and loaded into a ram extruder, resulting in a relatively thick-walled tube extrudate (typically having a wall thickness in the range of 0.010-0.017 inches (standard green tube for vascular graft)). Although the extrudate may be slightly tensioned during take-up after extrusion, the extruded tube is “green”. (As used herein, a “green” fluoropolymer tube, such as a PTFE tube, is unsintered and either unexpanded or minimally expanded due to the manufacturing process such as, e.g., being subjected to tension during take-up after extrusion.) In addition, extruded green PTFE typically contains a lubricant, which is volatilized. Thereafter, the tube may be subjected to further processes, including heating, sintering and expansion, with expansion resulting in reducing the thickness of the tube wall to a range of about 0.010 to about 0.017 inches, with a range of about 100% to about 500% elongation. As a result, a seamless, thin-walled tube of ePTFE may be formed.
Conventional PTFE extrusion methods have had difficulty in producing thin-walled tubes for use as medical implants, such as vascular prostheses. For example, U.S. Pat. Nos. 5,700,285; 5,735,892 and 5,810,870 to Myers describe these difficulties, including the difficulty of maintaining the uniformity of wall thickness of thin-walled extruded PTFE tubes. Consequently, these patents are directed to producing thin-walled vascular tubes using thin PTFE sheets which are wrapped to form a tube.